The 18-year-old was sleeping in a tent at the Sziget music festival in Budapest when a tree branch weighing more than 100kg fell from a tree after it had snapped during a violent storm, Die Welt reported.
Efforts to revive the young actor were unsuccessful.
Henkel had a role in the television series Animals up to the Roof (Tiere bis unters Dach) and last year starred in the film adaption of the children's book The Black Brothers.
The actor had been living in Pécs, Hungary as an exchange student and Unesco ambassador this year.
He would have been 19 in October.